Optical communication is a technique that could play a central role in future broadband communications. For widespread use of the optical communication, the optical components used in optical communication systems are required to be higher in performance, smaller in size and lower in price. Optical communication devices using photonic crystals are one of the leading candidates for the next-generation optical communication components that satisfy the aforementioned requirements. Some of these devices have already been put into practical use, an example of which is a photonic crystal fiber for polarization dispersion compensation. Furthermore, recent efforts have had a practical goal of developing optical multiplexers/demultiplexers and other devices that can be used in wavelength division multiplexing.
A photonic crystal is a dielectric object having an artificial periodic structure. Usually, the periodic structure is created by providing the dielectric body with a periodic arrangement of modified refractive index areas, i.e. the areas whose refractive index differs from that of the body. Within the crystal, the periodic structure creates a band structure with respect to the energy of light and thereby produces an energy region in which the light cannot be propagated. Such an energy region is called the “photonic band gap (PBG)”. The energy region (or wavelength band) at which the PBG is created depends on the refractive index of the dielectric body and the period of the periodic structure.
Introducing an appropriate defect into the photonic crystal creates a specific energy level within the PBG (“defect level”), and only a ray of light having a wavelength corresponding to the defect level is allowed to be present in the vicinity of the defect. This means that a photonic crystal having such a defect will function as an optical resonator that resonates with light having a specific wavelength. Furthermore, forming a linear defect will enable the crystal to be used as a waveguide.
As an example of the above-described technique, Patent Document 1 discloses a two-dimensional photonic crystal having a body (or slab) provided with a periodic arrangement of modified refractive index areas, in which a linear defect of the periodic arrangement is created to form a waveguide and a point-like defect is created adjacent to the waveguide. This two-dimensional photonic crystal functions as the following two devices: a demultiplexer for extracting a ray of light whose wavelength equals the resonance frequency of the resonator from rays of light having various wavelengths and propagated through the waveguide and for sending the extracted light to the outside; and a multiplexer for introducing the same light from the outside into the waveguide.
Including the one disclosed in Patent Document 1, many two-dimensional photonic crystals are designed so that the PBG becomes effective for either a TE-polarized light, in which the electric field oscillates in the direction parallel to the body, or a TM-polarized light, in which the magnetic field oscillates in the direction parallel to the body. According to this design, if a ray of light containing both kinds of polarized light is introduced into the waveguide or resonator of the two-dimensional photonic crystal, the propagating efficiency of the waveguide deteriorates, since one of the two kinds of polarized light diffuses within the crystal body. For example, if the periodic structure has a triangular lattice pattern and each modified refractive index area is circular (or cylindrical), the PBG will be effective for only the TE-polarized light. A waveguide or resonator formed in this two-dimensional photonic crystal causes negligible loss of TE-polarized light. However, it allows the TM-polarized light to freely propagate through the body and be lost, because no PBG is created for the TM-polarized light.
Taking the above problem into account, studies have been conducted on a new design of two-dimensional photonic crystal having a PBG for each of the TE-polarized light and the TM-polarized light in which the two PBGs have a common band. This band is called a “complete photonic band gap (complete PBG)” hereinafter. For example, FIG. 1 is a plan view of a two-dimensional photonic crystal disclosed in Non-Patent Document 1, which has a complete PBG created by periodically arranging triangular (or triangle-pole-shaped) holes 12 in a triangular lattice pattern in the slab 11. Within this two-dimensional photonic crystal, neither the TE-polarized light nor the TM-polarized light can leak from the waveguide, resonator or other device into the body as long as the wavelength of the light is within the complete PBG. Therefore, the propagating efficiency is maintained.
[Patent Document 1] Unexamined Japanese Patent Publication No. 2001-272555 ([0023]-[0027], [0032], FIGS. 1, and 5-6)
[Non-Patent Document 1] Hitoshi KITAGAWA et al. “Nijigen Fotonikku Kesshou Surabu Ni Okeru Kanzen Fotonikku Bando Gyappu (“Absolute Photonic Bandgap in Two-Dimensional Photonic Crystal Slabs)”, Preprints of the 50th Joint Symposia on Applied Physics, Japan Society of Applied Physics, March 2003, p. 1129